Conventional microscope scanners scan microscope slides using a line charge coupled device (CCD) of fixed width typically in the order of about 1.28 mm when scanning at 40× magnification. To accommodate scanning of slides greater than 1.28 mm wide a slide has to be scanned in multiple, slightly overlapping swathes. The scanner has the ability to maintain focus dynamically as a scan progresses but in order to do so it requires an initial seed focus for a given swathe. An autofocus procedure is responsible for determining the initial seed focus position for a given swathe to initialise the dynamic focus tracking system. The autofocus procedure however, adds time delays which increases the scanning time and, therefore, reduces the productivity of the scanner.
Further, biological specimens typically have circular or elliptical structures which have little detail at the edges of an image. Thus conventional scanning from one edge of the slide to the next is unlikely to work effectively as prediction errors from the dynamic focus procedure are likely to be large from, for example, the first swathe to the second swathe as the first swathe may have little or no detail.
It is, therefore, desirable to have an improved scanning method that increases the probability of getting a successful scan of a slide in focus and to reduce the number of autofocus steps required per sample.